This invention relates to a fuel injection pump for an internal combustion engine, such as a diesel engine.
In diesel engines, a fuel injection pump periodically supplies fuel to cylinders or combustion chambers. As the fuel injection pressure rises, the injected fuel is atomized better and the engine performance is enhanced.
Some distribution-type fuel injection pumps have a plunger which is driven to reciprocate synchronously with the engine crankshaft rotation by means of a cam mechanism. The end face of the plunger defines in part a pumping chamber. As the plunger moves in one direction, the pumping chamber contracts and thus fuel is driven out of the pumping chamber and is injected into the engine cylinder. As the plunger moves in the opposite direction, the pumping chamber expands and thus fuel is drawn into the pumping chamber.
The cam mechanism includes rollers and cam protrusions urged into engagement with the rollers by a cam spring. The cam protrusions are fixed to the plunger. As the engine crankshaft rotates, the cam protrusions move relative to the rollers, reciprocating the plunger.
The profile of the cam protrusions is generally designed such that the acceleration of the plunger changes in direction near the center of the fuel injection stroke. As a result of this change of the acceleration direction, the considerable inertia of the cam protrusions and the plunger acts against the force of the cam spring in the latter half of the fuel injection stroke. If the inertia overcomes the spring force, the cam protrusions move out of engagement with the rollers, impairing the fuel injection characteristics. Accordingly, the force of the cam spring is chosen to be adequate to hold the cam protrusions in constant engagement with the rollers.
The fuel injection pressure is proportional to the height or lift of the cam protrusions. If the lift of the cam protrusions is set to be great in order to achieve a high fuel injection pressure, a strong cam spring is necessary to hold the rollers and the cam protrusions in continuous contact. In this case, the cam protrusions are periodically pressed against the rollers by strong forces so that their service lives are shortened. Furthermore, a correspondinly larger torque to is required to drive the cam mechanism.